Heavenly Merman netting unlimited bream (8)
I believe the answer is:
ethereal
'heavenly' is the definition.
(similar in meaning)
'merman netting unlimited bream' is the wordplay.
'merman' becomes 'ethel' (I've seen this before).
'netting' means one lot of letters goes inside another (net can mean to contain or capture).
'unlimited' says to take the centre (discard the edges or 'limits' of the word).
The central letters of 'bream' are 'rea'.
'ethel' placed around 'rea' is 'ETHEREAL'.
(Other definitions for ethereal that I've seen before include "Insubstantial or not of this world" , "Delicate and perfect" , "Exquisite - as light as air" , "gossamer" , "Refined, airy" .)